1. Field of the Invention
The present invention relates to an electric power generation device and to an electronic instrument using the same.
2. Background Art
Heretofore, an electric power generation device of an electrostatic induction type has been known, in which strap-like electrets and comb teeth-like operation electrodes are arranged opposite to each other, and the operation electrodes vibrate in a horizontal direction with respect to the electrets, whereby electric power generation by electrostatic induction is enabled (for example, refer to Japanese Patent Laid-Open Publication No. 2006-180450).
In this electric power generation device, when electrets 3 and operation electrodes 1 come close to each other as shown in FIG. 9A, electric charges are induced to the operation electrodes 1. Then, when the operation electrodes 1 and the electrets 3 are moved relatively to each other in the horizontal direction from this state, then as shown in FIG. 9B, an induced electric charge amount of the operation electrodes 1 is changed, and an amount of such a change is outputted as a current to an external circuit (load R).
However, in the case of the above-described conventional technology, the electrets 3 are formed on an underlying electrode 2, and accordingly, a major part of electric flux lines emitted from the electric charges of the electrets 3 goes toward the underlying electrode 2. Hence, only a very small part of the electric flux lines goes toward the operation electrodes 1 as important targets, and accordingly, the electric charges induced to the operation electrodes 1 are very small in comparison with those induced to the underlying electrode 2, and there has been a problem that surface electric charges of the electrets 3 cannot be utilized effectively.
Specifically, as shown in FIG. 10, when a surface electric charge density of the electrets 3 is σ, a density of the electric charges induced to the operation electrodes 1 is σ1, and a density of the electric charges induced to the underlying electrode 2 is σ2, then the densities of the electric charges induced to the operation electrodes 1 and the underlying electrode 2 in the event where the operation electrodes 1 and the electrets 3 come closest to each other and are oppose to each other right in front are represented individually by the following formulae:σ1=−σ·(d2′)/(d1+d2′)σ2=−σ·(d1)/(d1+d2′)d2′=d2/∈s 
(∈s is a relative dielectric constant of the electrets)
A usual film thickness d2 of the electrets 3 is a whole lot smaller in comparison with a distance d1 thereof with the operation electrodes 1. For example, when d2 is equal to 10 μm, d1 is equal to 100 μm, and ∈s is equal to 2, then in the atmosphere, σ1 becomes substantially equal to −0.05σ, and σ2 becomes substantially equal to −0.95σ. In this example, the electric charges induced to the operation electrodes 1 becomes approximately 1/19 of the electric charges induced to the underlying electrode 2.
As discussed above, in comparison with the electric charges induced to the underlying electrode 2, the electric charges which are induced to the operation electrodes 1 and actually contribute to power generation are small, and the surface electric charges of the electrets 3 are not utilized effectively.
Moreover, the maximum electric power Pmax that can be taken out from this power generation device is represented by the following general formula (1), and an output thereof is proportional to a square of the surface electric charge density of the electrets 3.
Furthermore, the surface electric charge density σ of the electrets 3 is represented by the following general formula (2), and is proportional to a surface potential V of the electrets 3.
Therefore, an electric power generation amount is increased as the surface potential of the electrets 3 is being enhanced.
                              P          max                =                                                            σ                2                            ·              n              ·              A              ·              2                        ⁢            π            ⁢                                                  ⁢            f                                                                                ɛ                  s                                ·                                  ɛ                  0                                                            d                2                                      ⁡                          [                                                                                          ɛ                      s                                        ·                                          d                      1                                                                            d                    2                                                  +                1                            ]                                                          (        1        )                                σ        =                              ɛ            s                    ·                      ɛ            0                    ·                      V                          d              2                                                          (        2        )            
σ: surface electric charge density of electrets
n: number of poles, that is, number of electrets
A: maximum area where operation electrodes and electrets overlap each other
f: frequency of reciprocating motion of operation electrodes
d2: thickness of electrets
d1: distance between electrets and operation electrodes
∈s: relative dielectric constant of electrets
∈0: relative dielectric constant of vacuum
V: surface potential of electrets
However, in the case where the electrets 3 are thinned into such a strap shape and a wiring density thereof is enhanced as in the conventional technology, then in the event of implanting electrons into the electrets 3 by corona discharge, an electric field of end portions of the strap-like electrets 3 is intensified by an edge effect, and the electrets 3 repel the electrons. Hence, it becomes difficult to charge the electrets 3, and there has been a problem that it is difficult to increase the electric power generation amount since a high surface potential cannot be applied to the electrets 3.
Moreover, problems described as follows have been inherent in the electret electric power generation device (electric power generation device of electrostatic induction type) of the conventional technology.                Since the operation electrodes 1 are formed into the comb teeth shape, a surface area of surfaces of the operation electrodes 1 which generate the induced electric charges, the surfaces being opposed to the electrets 3, is reduced, and electric power generation efficiency with respect to a device area is poor.        It is necessary to draw wires from the operation electrodes 1 which move in a vibration manner, and a device structure becomes complicated in order to stably take out electric power.        As understood from the general formulae (1) and (2), a narrower gap (d1) between the electrets 3 and the operation electrodes 1 brings about a large effect for the electric power generation. However, the narrower the gap is, the larger the electrostatic suction force between the electrets 3 and the operation electrodes 1 becomes, and accordingly, a mechanism comes to be required, which maintains the gap accurately so that the electrets 3 and the operation electrodes 1 cannot contact each other. Therefore, the device structure becomes complicated.        